Chief38 and others covered most of it. Sideplate removal is not for everyone, and if you are not mechanically inclined and willing to learn, you should leave it to a good gunsmith. For reasons stated above, every new gun should be properly cleaned by a fully competent gunsmith. Every used gun, moreso.

I did once detail disassemble a used Detective Special that didn't need it. That gun looked like it had been cleaned by a gunsmith before giving it to his son who was a cop. Because it had. All other guns I cleaned needed it.

How often a gun needs cleaning depends on use. A properly lubed revolver that has sat for decades unused doesn't need cleaning. A moderately used revolver may need internal cleaning every few years or so, if that. A gun carried in the pocket, even inside a holster, may need internal cleaning yearly, as will a gun used to shoot thousands of rounds.